Add an "api signature" to summaries.

pkg/analyzer/lib/src/summary/flat_buffers.dart

Index: pkg/analyzer/lib/src/summary/flat_buffers.dart
diff --git a/pkg/analyzer/lib/src/summary/flat_buffers.dart b/pkg/analyzer/lib/src/summary/flat_buffers.dart
index 81bd2dbbb70a7a44dcb98a459b668bcd39f32833..3dd0991479469865fe15126692bd714baea3b4b7 100644
--- a/pkg/analyzer/lib/src/summary/flat_buffers.dart
+++ b/pkg/analyzer/lib/src/summary/flat_buffers.dart
@@ -9,6 +9,136 @@ import 'dart:convert';
 import 'dart:math';
 import 'dart:typed_data';
 
+import 'package:convert/convert.dart';
+import 'package:crypto/crypto.dart';
+
+/**
+ * An instance of [ApiSignature] collects data in the form of primitive types
+ * (strings, ints, bools, etc.) from a summary "builder" object, and uses them
+ * to generate an MD5 signature of a the non-informative parts of the summary
+ * (i.e. those parts representing the API of the code being summarized).
+ *
+ * Note that the data passed to the MD5 signature algorithm is untyped.  So, for
+ * instance, an API signature built from a sequence of `false` booleans is
+ * likely to match an API signature built from a sequence of zeros.  The caller
+ * should take this into account; e.g. if a data structure may be represented
+ * either by a boolean or an int, the caller should encode a tag distinguishing
+ * the two representations before encoding the data.
+ */
+class ApiSignature {

[scheglov, 2016/08/04 22:50:08]

Until now this file was pretty much independent of summaries and was just a
general purpose flat-buffers implementation for Dart. Could we move this class
out of this file?

[Paul Berry, 2016/08/04 23:05:46]

Done.

+  /**
+   * Version number of the code in this class.  Any time this class is changed
+   * in a way that affects the data collected in [_data], this version number
+   * should be incremented, so that a summary signed by a newer version of the
+   * signature algorithm won't accidentally have the same signature as a summary
+   * signed by an older version.
+   */
+  static const int _VERSION = 0;
+
+  /**
+   * Data accumulated so far.
+   */
+  ByteData _data = new ByteData(4096);
+
+  /**
+   * Offset into [_data] where the next byte should be written.
+   */
+  int _offset = 0;
+
+  /**
+   * Create an [ApiSignature] which is ready to accept data.
+   */
+  ApiSignature() {
+    addInt(_VERSION);
+  }
+
+  /**
+   * For testing only: create an [ApiSignature] which doesn't include any
+   * version information.  This makes it easier to unit tests, since the data
+   * is stable even if [_VERSION] is changed.
+   */
+  ApiSignature.unversioned();
+
+  /**
+   * Collect a boolean value.
+   */
+  void addBool(bool b) {
+    _makeRoom(1);
+    _data.setUint8(_offset, b ? 1 : 0);
+    _offset++;
+  }
+
+  /**
+   * Collect a sequence of arbitrary bytes.  Note that the length is not
+   * collected, so for example `addBytes([1, 2]);` will have the same effect as
+   * `addBytes([1]); addBytes([2]);`.
+   */
+  void addBytes(List<int> bytes) {
+    _makeRoom(bytes.length);
+    new Uint8List.view(_data.buffer)
+        .setRange(_offset, _offset + bytes.length, bytes);
+    _offset += bytes.length;
+  }
+
+  /**
+   * Collect a double-precision floating point value.
+   */
+  void addDouble(double d) {
+    _makeRoom(8);
+    _data.setFloat64(_offset, d, Endianness.LITTLE_ENDIAN);
+    _offset += 8;
+  }
+
+  /**
+   * Collect a 32-bit unsidneg integer value.

[scheglov, 2016/08/04 22:50:08]

unsigned

[Paul Berry, 2016/08/04 23:05:46]

Wow, that's a pretty bad transposition even by my standards!  Fixed.

+   */
+  void addInt(int i) {
+    _makeRoom(4);
+    _data.setUint32(_offset, i, Endianness.LITTLE_ENDIAN);
+    _offset += 4;
+  }
+
+  /**
+   * Collect a string.
+   */
+  void addString(String s) {
+    List<int> bytes = UTF8.encode(s);
+    addInt(bytes.length);
+    addBytes(bytes);
+  }
+
+  /**
+   * For testing only: retrieve the internal representation of the data that
+   * has been collected.
+   */
+  List<int> getBytes_forDebug() {
+    return new Uint8List.view(_data.buffer, 0, _offset).toList();
+  }
+
+  /**
+   * Return a hex-encoded MD5 signature of the data collected so far.
+   */
+  String toHex() {
+    return hex.encode(
+        md5.convert(new Uint8List.view(_data.buffer, 0, _offset)).bytes);
+  }
+
+  /**
+   * Ensure that [spaceNeeded] bytes can be added to [_data] at [_offset]
+   * (copying it to a larger object if necessary).
+   */
+  void _makeRoom(int spaceNeeded) {
+    int oldLength = _data.lengthInBytes;
+    if (_offset + spaceNeeded > oldLength) {
+      int newLength = 2 * (_offset + spaceNeeded);
+      ByteData newData = new ByteData(newLength);
+      new Uint8List.view(newData.buffer)
+          .setRange(0, oldLength, new Uint8List.view(_data.buffer));
+      _data = newData;
+    }
+  }
+}
+
 /**
  * Reader of lists of boolean values.
  *